Percentage of the U.S. population that will get the flu, on average, each year: between 5% and 20%.

Number of Americans hospitalized each year because of flu complications: 200,000, on average.

The number of people who die each year from flu-related causes in the U.S.: ranges from 3,000 to 49,000.

The typical incubation period for the flu is one to four days. Adults can be contagious from the day before symptoms begin through five to 10 days after the illness starts.

Number of flu vaccine doses available in the U.S. for the 2013-2014 flu season: Between 135 and 139 million.

The CDC recommends that everyone over six months of age get a flu vaccine as soon as it's available.

Flu activity usually peaks in January and February.

During 2009-2010, a new and very different flu virus (called H1N1, or swine flu) spread worldwide, causing the first flu pandemic -- global outbreak of disease caused by a new flu virus -- in more than 40 years. It is estimated that the pandemic resulted in more than 12,000 flu-related deaths in the U.S. In contrast to seasonal flu, nearly 90% of the deaths occurred among people younger than 65.

The 2013-2014 trivalent influenza vaccine is made from the following three viruses:

an A/California/7/2009 (H1N1)pdm09-like virus;

an A(H3N2) virus antigenically like the cell-propagated prototype virus A/Victoria/361/2011;

a B/Massachusetts/2/2012-like virus.

The 2013-2014 quadrivalent vaccine containing two influenza B viruses include the above three viruses and a B/Brisbane/60/2008-like virus.

It takes about two weeks after vaccination for an adult to develop antibodies against the flu.

A regular case of the flu typically resolves after three to seven days for the majority of people, although cough and fatigue can persist for more than two weeks.